Determination of elevation of various points with dumpy level by collimation plane method and rise & fall method.

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AIM:

Determination of elevation of various points with dumpy level by collimation plane method and rise & fall method.

APPARATUS:

Dumpy level, leveling staff

Figure:-

THEORY:

Dumpy Level

Levelling:

The art of determining and representing the relative height or elevation

of different object/points on the surface of earth is called leveling. It deals with measurement in vertical plane.

By leveling operation, the relative position of two points is known whether the points are near or far off. Similarly, the point at different elevation with respect to a given datum can be established by leveling.

LEVELLING INSTRUMENTS:-

The instrument which are directly used for leveling operation are:- Level, Levelling staff

Level: – An instrument which is used for observing staff reading on leveling staff kept over

different points after creating a line of sight is called a level.

The difference in elevation between the point then can worked out. A level essentially consists of the following points:

  1. Levelling Heads
  2. Limb plate
  3. Telescope

Telescope consists of two tubes, one slide into the other and fitted with lens and diaphragm having cross hairs. it creates a line of sight by which the reading on the staff is taken

The essential parts of a telescope are

  1. body 2) object glass 3)Eye-piece 4) Diaphragm 5) Ray shade 6) The rack and pinion arrangement 7) Focusing screw 8) Diaphragm screw.

Dumpy level:

The dumpy level is simple, compact and stable instrument. The telescope is rigidly fixed to its supports. Hence it cannot be rotated about its Longitudinal axis or cannot be removed from its support. The name dumpy is because of its compact and stable construction. The axis of telescope is perpendicular to the vertical axis of the level. The level tube is permanently placed so that its axis lies in the same vertical plane of the telescope but it is adjustable by means of captain head not at one end.

The ray shade is provided to protect the object glass. A clamp and slow motion screw are provided in modern level to control the movement of spindle,

about the vertical axis. The telescope has magnifying power of about thirty diameters.

The level tube is graduated to 2mm divisions and it has normally a sensitiveness of 20

seconds of are per graduation. The telescope may be internally focusing or external Focusing type.

Adjustment of the level

The level needs two type of adjustment

  1. Temporary adjustment and
  2. Permanent adjustment

Temporary adjustments of dumpy level

These adjustments are performed at each set-up the level before taking any observation.

    1. Setting up the level:- this includes
      1. Fixing the instrument in the tripod:- the tripod legs are well spread on the ground with tripod head nearly level and at convenient height. Fix up the level on the tripod.
      2. Leg adjustment:- Bring all the foot screws of the level in the centre of their run .Fix any two legs firmly into the ground by pressing them with hand and move the third leg to leg to right or left until the main bubble is roughly in the centre. Finally the legs is fixed after centering approximately both bubbles. This operation will save the time required for leveling.
    2. Levelling: – Levelling is done with the help of foot screws and bubbles. The purpose of levelling is to make the vertical axis truly vertical. The method of leveling the instrument depends upon whether there are three foot screws or four foot screws. In all modern instruments three foot screws are provided and this method only is described.

      1. Place the telescope parallel to pair of foot screws.
      2. Hold these two foot screw between the thumb and first finger of each hand and turn them uniformly so that the thumbs move either toward each other until the bubble is in centre.
      3. Turn the telescope through 90°so that it lies over the third foot screw.
      4. Turn this foot screw only until the bubble is centred.
      5. Bring the tescope back to its original position without reversing the eye piece and object glass ends.
      6. Again bring the bubble to the centre of its run and repeat these operation until the bubble remains in the centre of its run in both position which are at right angle to each other.
      7. Now rotate the instrument through 180°,the bubble should remain in centre provided the instrument is in adjustment: if not ,it needs permanent adjustment.

c) Focusing the eye piece:- To focus the eye piece, hold a white paper in front of the object glass ,and move the eye piece in or out till the cross hairs are distinctly seen. Care should be taken that the eye piece is not wholly taken out ,some times graduation are provided at the eye piece and that one can always remember the

particular graduation position to suit his eyes,This will save much time of focssing the eye piece.

(d) Focusing the object glass: – Direct the telescope to the leveling staff and on looking through the telescope, turn the focusing screw until the image appears clears and sharp. The image is thus formed inside the plane of cross hairs, Parallax,if any is removed by exact focusing. It may be noted that parallax is completely eliminated when there is no change in staff reading after moving the eye up and down.

Reduced Levels

The system of working out the reduced level of the points from staff reading taken in the field is called as reduced level (R.L) of a points is the elevation of the point with reference to the same datum.

There are two systems of reduced levels

  1. The plane of collimation system (H.I. method)
  2. The Rise and fall system
  3. The plane of collimation system (H.I. method)

In this system, the R.L. of plane of collimation (H.I) is found out for every set-up of the level and then the reduced levels of the points are worked out with the respective plane of collimation as described below.

    1. Determine the R.L. of plane of collimation for the first set up of the level by adding B.S. to the R.L. of B.M. i.e( R.L of plane of collimation= R.L. of B.M.+B.S.)
    2. Obtained the R.L. of the intermediate points and first change point by subtracting the staff readings (I.S. and F.S. from the R.L. of plane of collimation (H.I). (R.L. of a point=R.L of plane of collimation H.I.-I.S or F.S)
    1. When the instrument is shifted and set up at new position a new plane of collimation is determined by addition of B.S. to the R.L of change point. Thus the levels from two set-ups of the instruments can be correlated by means of B.S. and F.S. taken on C.P.
    2. Find out the R.L.s of the successive points and the second C.P. by subtracting their staff readings from this plane of collimation R.L.
    3. Repeat the procedure until all the R.Ls are worked out.

Observation table:-

Station Reading R.L. of plane collimation (H.I) Reduced Level Remarks
B.S I.S F.S

Arithmetical check: The difference between the sum of the back sights and the sum of the fore sights should be equal to the difference between the last and first reduced levels.

i.e ∑B.S – ∑ F.S.= LAST R.L –FIRST R.L

  1. The Rise and fall system

In this system, there is no need to determine R.L. of plane of collimation .The difference of level between consecutive points are obtained as described below.

    1. Determine the difference in staff readings between the consecutive point comparing each point after the first with that immediately proceeding it.
    2. Obtained the rise or fall from the difference of their staff reading accordingly to the staff reading at the point is smaller or greater than that of proceeding point.
    3. Find out the reduced level of each point by adding the rise to or subtracting fall from the R.L. of a proceeding point.

Observation table:-

Station Reading Rise Fall Reduced Level Remarks
B.S I.S F.S

Arithmetic check:- The difference between the sum of back sight and the sum of fore sight= difference between the sum of rise and the sum of fall = the difference between the last R.L. and the first R.L.

∑B.S-∑F.S =∑RISE -∑FALL =LAST RL- FIRST RL

Inverted staff reading

When the B.M of staff station is above the line of collimation (or line of sight) the staff is held inverted on the point and reading is taken .This reading being negative is entered in the level field book with minus sign, or to avoid confusion, ‘Staff inverted’ should be written in the remarks column against the entry of the reading.

The results are tabulated as below:

B.S. I.S F.S H.I R.L Remarks
-2.795 97.215 100.000 B.M.Staff
inverted
1.500 95.715 Point A

When the reading on the inverted staff is a foresight or intermediate sight .it should also be recorded in field book with minus sign

The R.L. of such points may be worked at as: R.L.of the point (where the inverted staff is held)

=R.L. of H.I +F.S. or I.S.reading

RESULT:

The various reduced levels are calculated by rise and fall method and by using height or plane pf collimation method and are shown in observation table.

 

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